The epigenetic mechanism of Alzheimer disease（ AD） is attracting increasing attention， and 5-hydroxymethylcytosine involved DNA methylation is an important means of epigenetic modification. Recent studies believe that 5-hydroxymethylcytosine is involved in the occurrence and development of AD. This is a very valuable reference for the further revelation the pathogenesis of AD. This article reviews research on the relationship between 5-hydroxymethylcytosine and AD.